A two-wire telephone set includes a base unit connected to a central office of a telephone service provider via a two-wire telephone line and also includes a handset connected to the telephone base via a four-wire handset cable. The handset cable has four wires because, for two-way voice communication, the handset includes both a microphone and a speaker, each of which requires a pair of wires. Typically, the telephone base supplies audio signals to the speaker and a DC biasing voltage to the microphone, while the telephone base receives audio signals from the microphone. A two-wire to four-wire converter included in the telephone set converts the two central office signals into the four handset signals. In addition, the telephone set includes a ring detector for detecting an AC ring signal provided by the central office and a hook-switch for signalling the central office of the on-hook/off-hook status of the telephone set. The hook switch controls draw of DC current from the central office by the telephone set, which is sensed by the central office.
A conventional modem transmits digital data over a two-wire telephone line by modulating an analog carrier signal according to the digital data. Typically, the digital data is generated by a computer or facsimile machine connected to the modem. The carrier signal is a tone within the frequency range of telephone transmission line. Upon reception by a second modem at the other end of the transmission line, the digital data is reconstructed by demodulating the received signal.
Business organizations often utilize a private branch exchange (PBX) for providing telephone service to telephone users within the organization. Telephones located on the desks of the users are each connected to the PBX while the PBX is connected to a telephone service provider via one or more outside lines. The PBX typically includes capability for appropriately connecting incoming calls to the user telephone sets and for connecting outgoing calls from a user telephone set to an outside line. In this way, fewer than one outside line per user telephone is needed, thus, reducing the cost of the telephone service. In addition, the PBX typically provides a variety of features to the users of the PBX, such as connecting calls among the users and providing voicemail services.
To implement all of the functions of the PBX, certain control and overhead communications must take place between each user's telephone set and the PBX. These communications typically include digital status and command signals in addition to the two-way voice signals necessary to carry on a telephone conversation. For example, the PBX must know whether a telephone set is connected to a particular extension in order to know whether or not to route calls to that extension. As another example, the PBX must interact with the user telephone sets in order for the users to initiate and terminate telephone calls and to access voicemail and other features of the PBX.
In general, the protocol utilized for control and overhead communications differs among the various manufacturers of PBX's. In addition, the voice signals can be digitally sampled and compressed according to various different companding schemes (e.g. .mu.-law or A-law). Therefore, a two-wire analog telephone instrument, such as modem, fax modem, facsimile machine or teleconferencing device, cannot generally interface directly with the PBX without a dedicated analog port from the PBX.
This creates a problem for users of a PBX who wish to use universally available analog telephone instruments, such as modems, fax modems, facsimile machines, teleconferencing devices, headsets or handsets, in addition to their PBX-compatible telephone sets. This problem has intensified by a recent increase in demand for access to the world wide web, which is typically accessed through use of a modem connected to a personal computer. A proposal has been to provide a dedicated outside line for each such analog telephone instrument. This solution is not entirely satisfactory, however, because it negates the savings which result from the PBX limiting the number of required outside lines. Another solution has been to provide an analog line card in the PBX and a separate line connecting the two-wire analog telephone instrument to the PBX. This solution can be costly due to the need to install separate extension lines to connect each of the PBX-compatible telephone set and the analog telephone instrument to the PBX. Further, the addition of analog line cards can necessitate a PBX that is has a higher capacity, and hence, higher cost, that would otherwise be required.
Another solution has been to provide a device which interfaces a modem with a telephone set through the handset port of the telephone set. For example, U.S. Pat. No. 4,907,267 discloses a modem interface device for use with a telephone set having a base unit and a handset. The telephone set can be a two-wire telephone set or a telephone set designed for use with a PBX. To use the modem interface device, the handset is unplugged from the handset jack of the base and plugged into a handset jack in one end of the device. Extending from the device is a four-wire cable which is connected to the handset jack of the base. The device also includes a modular jack for accepting a two-wire cable which connects the device to a two-wire telephone instrument, such as a modem. A series of switches are manually positioned to select between voice and data communications and to configure the interface device to match the signalling characteristics of the particular telephone set being used.
The manually operable switch arrangement described in U.S. Pat. No. 4,907,267 is improved upon in two products manufactured by Unlimited Systems Corp. of San Diego, Calif. A first of these products, the "KONEXX Office Konnector," connects to the base of a telephone set and to the handset to provide an interface for a two-wire telephone, facsimile machine or modem. The device detects when the two-wire telephone, facsimile machine or modem is placed off-hook for switching between voice and data communications. A second of the these products, the "KONEXX Konference," is similarly connected between the base and handset, but provides an interface for a teleconferencing device. For each of these devices, a manually operable switch is positioned in one of four positions for adjusting the device to the signalling characteristics of the particular telephone set being used.
A drawback to the aforementioned interface devices is that the switch positions may be incorrectly set. In addition, there is a likelihood of encountering a telephone set having signalling characteristics that cannot be met by these interface devices. For example, the bias voltage level and AC signal coupling characteristics of the microphone connection to the telephone base can vary from manufacturer to manufacturer as well as the output impedance and signal levels provided via the handset speaker connection. Also, the wire assignments within the handset cable can vary. A single return may be provided for both the microphone and speaker or each may have two dedicated wires. Failure of an interface device to properly match the signalling characteristics of the telephone set may result in inoperability or lost data and may result in the frustration of users of such devices who may not have the technical ability, nor the inclination, to resolve such problems.
What is needed is a telephone handset interface that has sufficient flexibility to match the signalling characteristics of a wide variety of commercially available telephone sets while minimizing technical ability required from a user of such interface.